The present invention relates to a scanning electron microscope that scans the surface of a specimen with an electron beam and forms a two-dimensional electron image representing the shape or composition of the surface of the specimen through the detection of secondary signals produced by the specimen. More particularly, the present invention relates to a scanning electron beam microscope suitable for forming electron beam images of a high resolution at a high throughput by rapidly moving an observation point to tens of test positions on a semiconductor wafer as a specimen.
A scanning electron microscope (hereinafter abbreviated to “SEM”) accelerates electrons emitted by an electron source of a heating electron emission type or a field electron emission type, collimates the accelerated electrons in a fine electron beam, i.e., a primary electron beam using an electrostatic lens or a magnetic field lens, scans a specimen two-dimensionally with the primary electron beam, detects secondary electrons generated by the specimen irradiated with the primary electron beam or secondary signal electrons, i.e., reflected electrons, and forms a two-dimensional electron image by applying intensities of detection signals as brightness modulating inputs to a cathode-ray tube (abbreviated to “CRT”) that is scanned in synchronism with a scanning operation using the primary electron beam.
Device miniaturization has progressively advanced in the semiconductor industry in recent years, and optical microscopes for inspection in semiconductor device fabricating processes and test processes have been replaced by SEMs. The SEM uses an electron beam for dimension measurement and testing electrical operations. When observing an insulating specimen, such as a wafer that is used in the semiconductor industry, is observed with a SEM, a low acceleration voltage of 1 kV or below must be used not to charge the insulating specimen. Generally, the resolution of a general SEM using a low acceleration voltage of 1 kV is about 10 nm. As the miniaturization of semiconductor devices advances, demand for SEMs capable of forming images in a high resolution by using a low acceleration voltage has increased. A retarding system and a boosting system were developed and proposed in, for example, Japanese Patent Laid-open No. Hei 9-171791 to meet such demand. Those previously proposed systems enable observation in a resolution of about 3 nm under optimum conditions for observation.